More specifically the present invention concerns coin validation using the phase shift caused by a coin to be validated which occurs when electromagnetic radiation is applied to the coin. A desirable feature of systems employing such a technique is that the phase shift, for a given frequency of radiation, is dependent mainly on the thickness and material of the coin and only to a small extent, if at all, on the lateral position of the coin relative to the validating apparatus or on features of the validating apparatus itself.
An example of a known system of the above type is described in British Patent GB 1443945, in the name of the present applicant. In this system a coin to be tested is caused to pass between a transmitting coil and a receiving coil. An oscillator supplies the transmitting coil with low-frequency, e.g. 320 kHz, electromagnetic radiation. The receiving coil receives electromagnetic radiation which, as a result of the presence of the coin, is phase-shifted with respect to the oscillator output. The phase of the oscillator is compared with that of the output signal from the receiving coil, and the phase difference is compared with a reference value to determine whether or not the coin is deemed to be valid.
However, in the above system the magnitude of the phase shift at the receiving coil is dependent on the oscillator frequency. Thus, any instability in the oscillator frequency will result in a different phase change to be expected for a valid coin.
In order to compensate for this effect, it is possible to measure the oscillator frequency and to determine a suitable phase shift reference value in dependence on the measured frequency. Such a system is described in EP-A2-0110510, also in the name of the present applicant. Since the phase shift is dependent, for a given coin, mainly on the oscillator frequency, this system has the advantage that it is merely necessary to measure the frequency in addition to performing the phase comparison to obtain a reliable validation. In particular, no adjustments are needed to compensate for tolerance of the components. However, for each value of measured frequency, an appropriate phase shift reference value must be generated, which will involve either the need for storing a table of frequencies and corresponding phase shift values or, alternatively, the need for the system to run an algorithm program for converting frequency values into corresponding values of phase shift.
It would be desirable to provide a system for coin validation based on the phase shift across a coin to be tested, and thereby exhibiting the advantages of the above-mentioned systems, but which does not produce errors in validation arising from variations in oscillator frequency, and does not require the provision of stored values for the frequency and phase shift correlation or the storage and execution of an algorithm for determining a reference phase shift Value from a given measured frequency.
Furthermore, it would be desirable to provide such a system which is not limited to a single frequency, so that the full range of coin thicknesses and conductivities can be encompassed.